The present invention relates to a semiconductor photodetector, and more particularly to a silicon-based semiconductor photodetector having a thickness-reduced optical waveguide layer including an optical absorption layer.
A conventional mesa-structure Si-based semiconductor photodetector will be described with reference to FIG. 1. A first Si cladding layer 11 is formed on an n-type silicon layer 3. The first Si cladding layer 11 has a recessed portion surrounding a mesa-portion. An optical absorption layer 4 is formed on the mesa-portion of the first Si cladding layer 11. A second Si cladding layer 11 is formed on the optical absorption layer 4. A silicon oxide film 7 is formed on the recessed portion of the first Si cladding layer 11 and the vertical side walls of the mesa-structure as well as a peripheral region of the top surface of the mesa structure. An aluminum electrode 9 is provided on the top surface of the mesa structure except for the peripheral region of the top surface of the mesa structure. This mesa-type Si-based semiconductor photodetector is disclosed in Journal of Light Wave Technology Vol. 12, No. 6, 1994, pp. 930-935.
A difference in refractive index between the n-type and p-type silicon layers 11 acting as the cladding layer and the Si/SiGe optical absorption layer 4 acting as a core layer is small. Silicon has a refractive index of about 3.5, whilst SiGe has a refractive index of about 3.6. Such the small difference in refractive index between the core layer 4 and the cladding layers 11 allows a remarkable permeation of the light into the cladding layers 11 in accordance with Goose-Henshen Shift Phenomenon. The depth of the permeation of the light into the cladding layers 11 is about 1 micrometer. In order to prevent the optical loss, each of the cladding layers 11 must have a thickness thicker than the depth of permeation of the light into the cladding layers 11, for example, more than 1 micrometer. This means that the optical waveguide layer comprising the cladding layers 11 and the optical absorption layer 4 must have a large thickness. This means that the mesa-structure has a large height or a large step.
A conventional planer Si-based semiconductor photodetector will be described with reference to FIG. 2. A silicon oxide film 2 is formed on an insulation substrate 1a. An n-type silicon layer 3 is formed on the silicon oxide film 2. A trench groove 10 is formed in the n-type silicon layer 3. Silicon oxide films 6 are formed on vertical side walls of the trench groove 10. An optical absorption layer 4 is selectively and epitaxially grown on the bottom surface of the trench groove 10 so that the optical absorption layer 4 is buried in the trench groove 10 except for an upper region of the trench groove 10. A p-type silicon layer 5 is subsequently and epitaxially grown on the optical absorption layer 4 so that a top surface of the p-type silicon layer 5 has the same level as the top surface of the n-type silicon layer 3. A silicon oxide film 7 is formed over the top surface of the n-type silicon layer 3 and a peripheral region of the p-type silicon layer 5. An aluminum electrode 9 is provided which extends over the p-type silicon layer 5 except for the peripheral region thereof. This planer-type Si-based semiconductor photodetector is disclosed in the Japanese laid-open patent publications Nos. 7-231113 and 7-52700 as well as in IEDM Technical Digest 1995, pp. 583-586.
A difference in refractive index between the n-type and p-type silicon layers 3 and 5 acting as the cladding layer and the Si/SiGe optical absorption layer 4 acting as a core layer is small. Silicon has a refractive index of about 3.5, whilst SiGe has a refractive index of about 3.6. Such the small difference in refractive index between the core layer 4 and the cladding layers 3 and 5 allows a remarkable permeation of the light into the cladding layers 3 and 5 in accordance with Goose-Henshen Shift Phenomenon. The depth of the permeation of the light into the cladding layers 3 and 5 is about 1 micrometer. In order to prevent the optical loss, each of the cladding layers 3 and 5 must have a thickness thicker than the depth of permeation of the light into the cladding layers 3 and 5, for example, more than 1 micrometer. This means that the optical waveguide layer comprising the cladding layers 3 and 5 and the optical absorption layer 4 must have a large thickness. This means that it is difficult to do an epitaxial growth of the thick silicon layer.
In the above circumstances, it had been required to provide a novel mesa-structure Si-based semiconductor photodetector free from the above problems and also provide a novel planer Si-based semiconductor photodetector free from the above problems.